Ceramic fibers and a process for producing the same

ABSTRACT

Ceramic fibers characterized by comprising a quaternary system composed of Si, C, N, and O and having a specific resistance of 10 6  to 10 10  Ω·cm, and a suitable process for producing the ceramic fibers characterized by reacting infusible polycarbosilane fibers with ammonia and further heat treating the reaction product in an inert gas.

DESCRIPTION

1. Technical Field

The present invention relates to ceramic fibers and a process forproducing the same and more particularly to ceramic fibers comprising aquaternary system composed of Si, C, N and O, which are excellent inmechanical properties such as tensile strength and tensile modulus andin electrical properties such as electric resistance and permittivity,and it also relates to a process for producing the ceramic fibers.

2. Background Art

Conventional inorganic fibers obtained by using polycarbosilane asstarting material, include SiC fibers (see Japanese Patent Nos. 1217464and 1217465 and the like) and SiON fibers (see Japanese Patent Appln.Laid-Open No. (sho.) 61-12915 (12915/86)).

These conventional fibers have been used not only as reinforcing fibersfor a fiber-reinforced metal, a fiber-reinforced plastics and the like,but also as an electric insulating material, a heat-resistant materialand the like, because of their excellent properties.

Although the SiC fibers have excellent mechanical properties, they areelectrically disadvantageous in that they have a specific resistance of10³ to 10⁵ Ω·cm, they exhibit relatively large permittivity anddielectric loss when they are used in the form of a SiC fibers/resincomposite material and they have inferior radio wave transmittivity,whereby they are rendered unsuitable for use as a radome or the like.

On the other hand, SiON fibers are advantageous in that they have aspecific resistance exceeding 10¹⁰ Ω·cm, i.e., high electric insulatingperformance, exhibit lower relative permittivity and dielectric losswhen used in the form of a resin composite material than those of theSiC fibers and have radio wave transmittivity superior to that of theSiC fibers. However, the SiON fibers raise a problem that theirmechanical properties are inferior to those of the SiC fibers.

The primary object of the present invention is to solve theabove-described problems and to provide ceramic fibers having excellentelectrical and mechanical properties and a process for producing thesame.

DISCLOSURE OF INVENTION

The present inventors have made various studies with a view to solvingthe above-described problems and, as a result, have found that theabove-described problems can be solved by heat treating infusiblepolycarbosilane fibers in an ammonia gas atmosphere to nitride thefibers and further heat treating the thus obtained nitrided fibers in aninert gas or an atmosphere comprising 1 to 30% by volume of hydrogenchloride with the balance being an inert gas to obtain desired ceramicfibers. The present invention is based on this finding.

The ceramic fibers of the present invention are characterized bycomprising a quaternary system composed of Si, C, N, and O.

It is the most desirable that the contents of the elements, i.e., Si, C,N, and O, in the ceramic fibers of the present invention are 40 to 60%by weight, 0.2 to 30% by weight, 5 to 30% by weight, and 5 to 20% byweight, respectively. When the content of each of the elements is in theabove-described corresponding range, the ceramic fibers exhibitexcellent electrical and mechanical properties, i.e., a specificresistance of 10⁶ to 10¹⁰ Ω·cm, a tensile strength of 300 to 450 kg/mm²,a tensile modulus of 20 to 40 ton/mm², a relative permittivity of 3.0 to4.0 when in the form of an epoxy resin composite material (percentagevolume of fibers: 55%), and a dielectric loss of 0.02 or less when inthe form of said composite material.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flow sheet showing an example of the process for producingceramic fibers according to the present invention;

FIG. 2 is a graph showing the relationship between the content of eachcomponent and the temperature of heat treatment in an ammonia gas inExample 1;

FIG. 3 is a graph showing the relationship between each of the tensilestrength and tensile modulus, and the temperature of heat treatment inammonia gas in Example 1; and

FIG. 4 is a graph showing the relationship between the specificresistance and the temperature of heat treatment in ammonia gas inExample 1.

BEST MODE FOR CARRYING OUT THE INVENTION

A preferred embodiment of the process for producing ceramic fibersaccording to the present invention will now be described in detail withreference to the accompanying drawings.

FIG. 1 is a flow sheet showing an example of the process for producingceramic fibers according to the present invention. In FIG. 1, referencenumeral 1 designates an ammonia gas bomb, numeral 21 a bomb for an inertgas such as argon or nitrogen gas, numeral 22 a hydrogen chloride gasbomb, numerals 3, 41 and 42 each a gas blender, numerals 8, 91 and 92each a pressure regulator, numeral 10 a flow rate regulator, numeral 11a firing furnace, numeral 12 a vacuum pump, numeral 13 a manometer,numeral 14 a gas washing bottle, and numeral 15 a reaction tube.

In the process according to the present invention, polycarbosilanefibers obtained by continuous spinning are first made infusible byoxidizing the fibers with an oxidizing gas such as air or oxygen therebyto prevent the fibers from being fused in the second firing step whichwill be described later. The treatment for making the fibers infusiblemay be carried out, for example, by treating the fibers in the reactiontube 15 of the firing furnace 11 shown in FIG. 1, in an oxidizingatmosphere such as air or oxygen, at a temperature of 50° to 400° C. forseveral min. to 10 hr.

The infusible fibers thus obtained are then heat treated in an ammoniagas atmosphere at 100° to 600° C. for 0.5 to 6 hr to allow the fibers toreact with ammonia (this treatment being referred to as ("the firstfiring step"). In this step, the device for the treatment is evacuatedto a predetermined degree of vacuum with the vacuum pump 12 whilemonitoring the degree of vacuum with the manometer 13, and ammonia gasis supplied into the device from the ammonia gas bomb 1 through thevalve 6, the flowmeter 3 and the flow rate regulator 10 to form anammonia gas atmosphere within the device. The ammonia gas is regulatedto have a given flow rate with the flow rate regulator 10. The flow rateof the ammonia gas is preferably 10 to 500 ml/min.

After an ammonia gas atmosphere is formed within the device, theinfusible fibers are heat treated in the firing furnace 11 at atemperature of 100° to 600° C. When the heat treatment is conducted at atemperature below 100° C., no ceramic fibers having a sufficiently highnitrogen content can be produced. On the other hand, when the heattreatment is conducted at a temperature exceeding 600° C., ceramicfibers to be formed by the subsequent second firing will undesirablydegrade in mechanical properties such as tensile strength.

Then, the fibers thus heat treated in ammonia gas is further heattreated in an inert gas, such as nitrogen or argon gas, at a temperatureup to 1600° C. for 0.5 to 6 hr to prepare amorphous continuous ceramicfibers comprising Si, C, N, and O (this treatment being referred to as"the second firing step"). In this step, as in the first firing step,the device is evacuated to a predetermined degree of vacuum with thevacuum pump 12 while monitoring the degree of vacuum with the manometer13, and an inert gas is supplied into the device from the nitrogen orargon gas bomb 21 through the valve 51, the flowmeter 41 and the flowrate regulator 10 to form an inert gas atmosphere within the device. Theinert gas is regulated to have a given flow rate with the flow rateregulator 10. The flow rate of the inert gas is preferably 200 to 2500ml/min.

After an inert gas atmosphere is formed within the device, the fibersfrom the first firing step are heat treated in the firing furnace 11 ata temperature up to 1600° C. to obtain desired ceramic fibers. When theheat treatment is conducted at a temperature exceeding 1600° C.,particles of Si-N and Si-C in the fibers are grown into ones having anexcessively large size, whereby the resulting ceramic fibers willundesirably degrade in the strength.

Further, the above-described inert gas may be partly replaced withhydrogen chloride gas which is supplied from the hydrogen chloride gasbomb 22 through the valve 52 and the flow meter 42 into the gas blender7 to prepare a mixed gas consisting of 1 to 30% by volume of hydrogenchloride gas with the balance being the inert gas. The mixed gas soprepared may be flowed at a rate of 10 to 500 ml/min to conduct the heattreatment at a temperature up to 1600° C. In this case, the adjustmentof the hydrogen chloride content of the mixed gas enables the averagevalue and variation of the C content of the resulting ceramic fibers tobe controlled, which makes it possible to prepare homogeneous ceramicfibers. When the hydrogen chloride content of the mixed gas is less than1% by volume, no favorable effect can be attained. A hydrogen chloridecontent exceeding 30% by volume is also unfavorable because the N or Ccomponent in the fibers is reduced, so that the strength of the ceramicfibers obtained is lowered.

As described above, in the present invention, ceramic fibers comprisingSi, C, N, and O in respective predetermined amounts can be produced byreacting infusible polycarbosilane fibers with ammonia at 100° to 600°C. and further heat treating the reaction product in an inert gas or anatmosphere comprising 1 to 30% by volume of hydrogen chloride with thebalance being an inert gas at a temperature up to 1600° C. Said ceramicfibers have electrical and mechanical properties superior to those of thconventional SiC and SiON fibers.

EXAMPLES

The present invention will now be described in more detail withreference to the following Examples and Comparative Examples.

EXAMPLE 1

Polycarbosilane (average molecular weight of about 2000; melting pointof 220° to 230° C.) was melt spun and then made infusible at 180° C. for1 hr in air to obtain infusible polycarbosilane fibers. The infusiblefibers were heat treated in ammonia gas (at a flow rate of 200 ml/min)at 100° C., 400° C., and 600° C. for 1 hr to obtain samples heat treatedat different temperatures, respectively. Each of the samples was heattreated at 1200° C. for 1 hr in nitrogen gas (at a flow rate of 2000ml/min) to obtain three kinds of ceramic fibers.

COMPARATIVE EXAMPLE 1

The same infusible polycarbosilane fibers as those used in Example 1were heat treated respectively at 650° C., 800° C., and 1000° C. for 1hr under similar conditions to those of Example 1 and further heattreated at 1200° C. for 1 hr in the same nitrogen atmosphere as that ofExample 1, thereby preparing three kinds of ceramic fibers heat treatedat the different temperatures especially in the first firing step.

The three kinds of ceramic fibers so obtained were each subjected toelemental analysis. The results are shown in FIG. 2. Similarly, theresults of measurements of the tensile strength and tensile modulus ofeach of the three kinds of the ceramic fibers are shown in FIG. 3, whilethose of measurements of the specific resistance of each thereof areshown in FIG. 4.

As is apparent from the results shown in FIG. 2, the ceramic fibers heattreated in ammonia gas at the temperatures exceeding 600° C. exhibitedthe saturated content of bound nitrogen, and the ceramic fibers heattreated at a temperature of 800° C. or higher in ammonia gas weresubstantially freed from bound carbon. Furthermore, as is apparent fromFIG. 3, the ceramic fibers subjected to the first firing at atemperature exceeding 600° C. had lowered tensile strength and tensilemodulus.

EXAMPLE 2

The ceramic fibers obtained in Example 1 were combined with aluminum bya high pressure casting method to produce a fiber-reinforced metal(FRM).

The FRM thus obtained had a percentage fiber volume of 40%, were freefrom voids, and had excellent adhesion between the fibers and thematrix. The tensile strength and tensile modulus at room temperature ofthe FRM were 90 to 100 kg/mm² and 11 to 12 ton/mm², respectively.

Further, the ceramic fibers were combined with an epoxy resin to producea fiber-reinforced plastics (FRP).

The relative permittivity of the resultant FRP was measured with theresult of 3 to 4 when the percentage fiber volume of the FRP was 55%.

EXAMPLE 3

The same polycarbosilane as that used in Example 1 was melt spun andthen made infusible in air at 180° C. for 1 hr to obtain infusiblepolycarbosilane fibers. The infusible fibers thus obtained were heattreated in ammonia gas (at a flow rate of 200 ml/min) at 450° C. for 2hr and further heat treated in a gas comprising 5% by volume of hydrogenchloride with the balance being nitrogen (at a flow rate of 2000 ml/min)at 1200° C. for 1 hr to obtain ceramic fibers.

The tensile strength and tensile modulus of the ceramic fibers thusobtained were measured and found to be 300 kg/mm² and 20 ton/mm²,respectively The ceramic fibers were subjected to elemental analysiswith the results that the Si, C, N and O contents were 53.2%, 8.4%,26.2% and 12.2%, respectively.

The specific resistance of the ceramic fibers was measured and found tobe 8×10⁶ Ω·cm. Further, a composite material comprising the ceramicfiber and an epoxy resin (the percentage fiber volume of the compositematerial: 55%) was prepared, and the relative permittivity anddielectric loss of the composite material were measured and found to be3.5 and 0.02, respectively.

EXAMPLE 4

The same infusible polycarbosilane fibers as those used in Example 1were heat treated in a furnace filled with ammonia gas at 600° C. for1.5 hr and further heat treated in a gas comprising 20% by volume ofhydrogen chloride with the balance being nitrogen at 1200° C. for 2 hrto obtain ceramic fibers. The tensile strength and tensile modulus ofthe ceramic fibers thus obtained were measured and found to be 250kg/mm² and 18 ton/mm², respectively. The ceramic fibers were subjectedto elemental analysis with the results that the Si, C, N and O contentswere 56.7%, 0.2%, 25,9% and 17.2%, respectively.

The analysis of the ceramic fibers by X-ray diffractometry revealed thatthe ceramic fibers were amorphous. The specific resistance of theceramic fibers were measured and found to be 6×10⁸ Ω·cm.

COMPARATIVE EXAMPLE 2

The tensile strength and tensile modulus of commercially available SiCfibers were measured and found to be 300 kg/mm² and 20 ton/mm²,respectively.

The specific resistance of the ceramic fibers was measured and found tobe 7.8×10³ Ω·cm. A composite material comprising the ceramic fibers andan epoxy resin (percentage fiber volume of 55%) was prepared, and therelative permittivity and dielectric loss thereof were measured andfound to be 5.2 and 0.2, respecively.

COMPARATIVE EXAMPLE 3

The same infusible polycarbosilane fibers as those used in Example 1were heat treated in a furnace filled with ammonia gas at a temperaturerise rate of 100° C./hr until the temperature reached a maximumtemperature of 800° C. and further fired in an argon gas atmosphere at atemperature rise rate of 100° C./hr and maintained at a maximumtemperature of 1200° C. for 1 hr, thereby obtaining ceramic fibers. Theceramic fibers were subjected to elemental analysis with the resultsthat the ceramic fibers were SiON fibers having Si, C, N, and O contentsof 57.2%, 0%, 29.5%, and 13.3%, respectively. The tensile strength andtensile modulus of the ceramic fibers were measured found to be 190kg/mm² and 14 ton/mm², respectively.

The specific resistance of the ceramic fibers was measured and found tobe 8.8×10¹⁰ Ω·cm. A composite material comprising the ceramic fibers anepoxy resin (percentage fiber volume of 55%) was prepared, and therelative permittivity and dielectric loss thereof were measured andfound to be 3.8 and 0.02, respectively.

Industrial Applicability

As described above, the ceramic fibers of the present invention haveexcellent mechanical properties equal to those of SiC fibers, highelectrical resistance and low relative permittivity comparable to thatof SiON fibers, i.e., high radio wave transmission performance, therebyrendering the ceramic fibers of the present invention very suitable foruse as reinforcing fibers for a high strength radome structure. Further,the process according to the present invention enables ceramic fibershaving excellent properties and comprising a quaternary system composedof Si, C, N, and O to be efficiently produced with a highreproducibility.

We claim:
 1. A process of producing ceramic fibers consistingessentially of a quaternary system composed of Si, C, N and O, whereinSi, C, N and O content of the ceramic fibers obtained is Si: 40 to 60%by weight, C: 0.2 to 30% by weight, N: 5 to 30% by weight and O: 5 to20% by weight, which consists of the steps of: (1) reacting infusiblepolycarbosilane fibers with ammonia at a temperature of 100° to 450° C.to obtain nitrogen containing fibers and (2) further heat treating saidnitrogen-containing fibers in an inert gas at a temperature up to 1,600°C.
 2. The process according to claim 1 wherein said infusiblepolycarbosilane fibers are prepared by reacting polycarbosilane fiberswith air or oxygen at a temperature of 50°-400° C. for a period of timeup to 10 hours.
 3. A process of producing ceramic fibers consistingessentially of a quaternary system composed of Si, C, N and O, whichconsists of the steps of 1) reacting infusible polycarbosilane fiberswith ammonia at a temperature of 100° to 450° C. to obtain nitrogencontaining fibers and 2) further heat treating said nitrogen-containingfibers in an inert gas wherein said inert gas contains 1 to 30% byvolume of hydrogen chloride.
 4. The process according to claim 3 whereinsaid infusible polycarbosilane fibers are prepared by reactingpolycarbosilane fibers with air or oxygen at a temperature of 50°-400°C. for a period of time up to 10 hours.